A mathematical model for bacterial growth is developed by consideration of the analogy between bacterial growth and a nonelementary chemical reaction. The resulting model is very general, including both substrate storage and basal metabolism. Under certain defined conditions it reduces to the known Monod-type and two-phase growth models. It predicts quantitatively the large substrate uptake rates seen in the contact stabilization process. In this model the concentrations of substrate and biomass are defined in terms of their carbon content as this approach gives greater conceptual and practical simplicity than conventional approaches based on BOD, COD, VSS, etc. Predictions of the model are found to agree with experimental data from a system of mixed denitrifying bacteria metabolizing a dilute dextrose substrate.